To Catch a Plutonium Thief, Try Antineutrinos

A new more secure technology for guarding against theft from nuclear reactors has passed its first test.

Researchers from Lawrence Livermore National Laboratory and Sandia National Laboratories in Livermore, Calif., successfully monitored the power output of a relatively small nuclear power reactor by measuring the number of antineutrinos—ghostly particles generated by nuclear fission—that struck a refrigerator-size tank of liquid.

Although the technology still has hurdles to overcome, it could pave the way for a new tool in efforts to prevent nuclear proliferation.

Nuclear reactors offer a potential source of weapons-grade materials to would-be bomb makers, particularly plutonium, which gradually accumulates in the uranium fuel rods that power a reactor.

As part of its work to prevent the spread of nuclear weapons, the International Atomic Energy Agency (IAEA), an organization established in 1957 by international treaty to promote peaceful uses of nuclear power, monitors and inspects nuclear reactors used for research as well as those that generate electricity.

Inspectors compare operators' records with its own monitoring data to assess whether the reactor could have produced excess plutonium beyond what the operators declared, says Julian Whichello, head of the IAEA's Novel Technologies Program.

Current monitoring instruments use sensors to monitor the amount of coolant used by the reactor. But that may require cables that are often expensive to install and maintain, and they expose the instruments to the risk of tampering, Whichello says.

Antineutrino detectors might ease these problems by directly measuring nuclear reactions. When uranium undergoes a fission reaction—the source of a reactor's energy—it splits into two or more lighter elements that rapidly decay, liberating an average of six antineutrinos per fission reaction.

The detectors consist of solids or liquids rich in protons that decay when hit by antineutrinos, causing dyes mixed in with the proton-rich medium to emit fluorescent light. Such detectors are attractive for nuclear monitoring, because with proper shielding to block out stray particles, "it's very hard to make signals that look like an antineutrino without a reactor," says Adam Bernstein, a Livermore staff scientist and deputy group leader of the Advanced Detectors Group.

Three years ago, at the request of IAEA, Bernstein and his colleagues at Livermore and Sandia installed a prototype detector at San Onofre Nuclear Generating Station (SONGS) in southern California, a 2,254-megawatt reactor. The instrument consisted of 1,400 pounds (640 kilograms) of mineral oil and the chemical 1,2,4-trimethylbenzene.

They placed the detector 30 feet (10 meters) underground to shield it from stray particles and 82 feet (25 meters) from the closer of two active reactor cores at the SONGS facility. They reported in the Journal of Applied Physics that their detector could measure the reactor's power level to within 3 percent accuracy, and they could tell within five hours whether operators had shut down the reactor. If inspectors know the power output of the reactors and its initial fuel, they can work out how much plutonium it should contain, Bernstein says.

The data is promising but applicable only to SONGS for the moment, says Andrew Monteith, an IAEA safeguards technology specialist. "The results at San Onofre show it could be a deployable instrument in certain circumstances," he says. Brazil, France, Canada and Russia all have plans to test antineutrino-based monitoring devices, he adds. The IAEA has scheduled a meeting in October to assess the cost and complexity of current detectors.

Bernstein says he and his colleagues are testing alternative detectors made of water or solid plastic. The mineral oil–based detector is relatively difficult to deploy, he says, and might pose a safety risk to IAEA inspectors from the 1,2,4-trimethylbenzene, which is toxic and flammable [see note below].

Another challenge might be the need to place the detector underground, which not all nuclear facilities may have space for, says Juan Collar, a physicist the University of Chicago who works on neutrino detection. Monteith notes that Brazil plans to test aboveground detectors.

And other, more experimental detectors might be easier to place in a reactor. Collar and Bernstein are both part of teams attempting to observe a different type of interaction between protons and antineutrinos that would require more sensitive detectors but would occur 100 to 1,000 times more frequently than in existing technologies, Collar says, allowing for smaller devices.

Correction (5/9/08): The original article implied that 1,2,4-trimethylbenzene would be in close proximity to the reactor core, which is not the case.

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